The present invention relates generally to a system for measuring conductivity or resistivity of a fluid and, more particularly, to such systems that further measure temperature and compensate for capacitance effects.
Conductivity/resistivity sensors have long been used to measure ionic concentration in target fluids. Measurement of conductivity or resistivity of a fluid provides useful information in varied industrial applications. Such measurements aid determining purity of fluid and detecting contamination or chemical concentration of fluid, in industrial processes across many industries such as pharmaceutical, food, chemical, wastewater treatment, and semiconductor manufacturing. Temperature sensing of the target fluid also provides useful information in industrial processes and can be used to compensate conductivity measurements based on the temperature of the fluid, since the conductivity of a given fluid is a function of temperature.
Conductivity sensors typically apply a potential difference across the target fluid, driving ionic current and forming a conductance cell in the target fluid. Conductivity sensors (or electrodes) are typically constructed as two metal pieces electrically connected by the fluid of interest. During measurement, an AC (alternating current) voltage (typically sine wave or square wave) is applied between the two plates and the current through the fluid is measured. Generally speaking, the ionic current through the target fluid is proportional to the applied potential difference, i.e. applied voltage (V). The resistance (R) of the target fluid equals the potential difference (V) over the current, (i.e., R=V/I). The conductance of the target fluid is defined as the reciprocal of the resistance. Ideally, electrolyte resistance, or conductivity would be easy to determine. In practice, however, such conductance cells often encounter additional electrical processes that must be considered.
Conductivity sensors for fluids commonly employ AC signals, such as sine wave or square wave, rather than DC signals. DC signals are problematic in that they tend to cause ion migration in the fluid and polarization, among other things. Such errors are exceedingly difficult to characterize with any accuracy, making DC signals impractical for such conductivity sensors. Thus, most conductivity sensors use AC signals having relatively low amplitude to minimize polarization.
With knowledge of the conductance cell and related parameters, ionic concentration and purity of the target fluid can be determined. To aid in calculating the desired parameters of the conductance cell, behavior of the conductance cell can be modeled by adopting an equivalent electrical circuit that imitates performance of the conductance cell.
With reference to FIG. 1, a schematic model of an equivalent electrical circuit is depicted for a common conductance cell, having two electrodes exposed to a target fluid and driven by an alternating current (AC). A list of equivalent components representing electrical process of the cell are provided, as follows:
R: Electrolyte Resistance
Rd: Faradic Impedance, or Electrode—Solution Interface Resistance
Cd: Double Layer Capacitance, or Electrode—Solution Interface Capacitance
Cp: Parallel Capacitance
Cw: Signal Lead or Wire Capacitance
Most commercially available electrodes have relatively high Cd (in the uF range). Normally, the driving frequency is chosen such that the equivalent impedance of Cd is very small (close to zero), allowing the AC current to bypass Rd. Thus, the effects from Cd and Rd can be minimized and in many applications neglected. A capacitive effect between the electrodes of the cell is commonly referred to as parallel capacitance (Cp). Parallel capacitance (Cp) is related to the geometry of the electrode and can be calibrated during development of the electronics.
However, capacitive effects of wiring used to connect the electrodes to other electronics of the system is commonly referred to as wire capacitance (Cw), and tends to contribute to measurement error. Wire capacitance tends to be difficult to characterize, particularly without detailed knowledge of parameters of the wire. Various parameters of wires or cables used in the sensor assembly, such as, length, materials used and wire gauge, impact the wire capacitance observed. Moreover, minimizing the length of the wire or cable reduces the capacitance effect. If the aforementioned parameters are known, wire capacitance can be characterized in some manner. It should be noted that the value attributable to wire capacitance is subject to aging of the wire and changes in temperature.
Current approaches for measuring conductivity often require a fixed cable length having known parameters connected to the electrode or, alternatively, a cable of minimal length to minimize the impact of capacitance attributable to the wiring extending between the electrode within the target fluid and the remaining electronics of the sensor assembly.
Although the aforementioned approaches are generally effective, shortfalls existed. For example, to maintain accurate measurements, the cable length must be known. A user cannot modify the length of the cable used, even if desired. Thus, a customer cannot use an electrode from other vendor, which may use different material, wire gauge and length; since high measurement errors may occur.
Some attempt to minimize capacitance effects by using complex circuitry and processes, which raises different issues. For example, some approaches attempt to pre-charge the cell prior to reading a measurement, in an attempt to minimize capacitance effects. Such approaches, however, require drive signals at comparatively low frequencies, which increase the risk of polarization, and require wiring or cables that are relatively short, and the user is limited in the length of cable that can be used.
It should be appreciated that there remains a need for a system for measuring conductivity of a fluid that enables use of relatively long wiring, or even unknown wire length, without adversely impact measurement accuracy. The present invention fulfills this need and others.